The basics: where everything begins and everything ends. From the onset of training through Black Belt and beyond, everything comes back to basics. You can’t practice them enough.
When you practice, do it properly. Doing 10 properly executed kicks is infinitely more valuable than doing 100 poorly executed kicks. You muscles need to learn how to perform the task and become accustomed to it. Start slowly with proper form and then work in some speed. Once your body knows how to accomplish the task slowly, doing it with speed will be easier.